1. Field of the Invention
The present invention relates generally to an optical fiber, and particularly to an optical fiber having a reduced viscosity mismatch between the core region and an adjacent cladding segment.
2. Technical Background
Conventional single-mode optical fibers typically have an SiO2 (silica) glass core region doped with a dopant suitable for raising the refractive index of the core region, and surrounded by a cladding glass of pure silica. A typical core region dopant is GeO2. The index difference between the core and the cladding is necessary to create a light guide wherein propagating light is generally confined to the core region. The concentration of GeO2 found in a conventional single-mode doped-core optical fiber may be in excess of 7 weight percent (wt. %). Because the high concentration of dopant is located in the core region of the optical fiber, the optical loss, or attenuation, of the optical fiber is higher than the attenuation expected in pure silica glass. To overcome the light absorbing characteristic of a core region containing relatively high concentrations of one or more dopants, pure silica core optical fibers were developed. That is, optical fibers having a core region composed of pure silica.
To create the refractive index difference between the core region and the cladding region in a pure silica core optical fiber, one or more refractive index-modifying dopants are added to the cladding region to reduce the refractive index of the cladding region to a value below the refractive index of the pure silica core region. For example, fluorine (F) is a commonly employed dopant to decrease the refractive index of the silica glass cladding region. The degree to which the refractive index of the cladding region of a pure silica core optical fiber is decreased below the refractive index of the core region depends upon the optical fiber design and the desired optical fiber parameters, but the addition of the index-modifying dopant to the cladding region rather than the core region eliminates the optical loss due to the presence of dopants in the core. Doping the cladding region also affects the viscosity of the cladding glass. That is, when a dopant such as F is added to a silica glass cladding region, the viscosity of the cladding region is lowered, resulting in a viscosity mismatch between the pure silica core glass and the doped silica cladding glass.
For a given set of draw conditions, one region of an optical fiber having a viscosity lower than the viscosity of another region results in the region of the optical fiber having the higher viscosity bearing more of the draw tension. For a pure silica core optical fiber this means the light-guiding core supports the tensile stress applied to the optical fiber during the draw process. The resulting stress may be retained within the optical fiber as residual stress. Residual stress, that is, stress that has been frozen into the fiber upon cooling from the draw temperature, is one cause of increased transmission loss. As a consequence, pure silica core optical fibers are typically drawn at very slow draw speeds, on the order of 1 meter per second, to minimize residual stress.
To mitigate the potential for increased residual stress, small amounts of chlorine (Cl) have been used as a core region dopant in otherwise pure silica core optical fibers in an effort to match the viscosity of the core glass to the viscosity of the cladding glass. However, the generally low level of chlorine that has been used, about 1 wt. % or less, has by itself been insufficient to adequately match the viscosity of the core region to the viscosity of the cladding region. Since Cl alone is not an effective modifier of viscosity, large amounts of Cl are required to closely match the core viscosity to the cladding viscosity. However, the high volatility of Cl limits the amount of Cl that can be added to the core region in some optical fiber manufacturing methods, such as those that employ outside vapor deposition (OVD) or vapor axial deposition (VAD). The inability to dope an adequate amount of Cl into the core region has limited the effectiveness of this approach, thus very slow draw speeds and high draw furnace temperatures are still typically required to avoid draw-induced defects such as voids in the glass, excess residual stress, and increased optical fiber attenuation in the manufacture of silica core optical fiber. Prior art optical fibers having predominantly pure silica cores continue to have high residual stresses—on the order of between about 50 MPa and 60 MPa.
Thus, there is a need for a means of more closely matching the viscosity between different regions of an optical fiber.